Abstract The airplane and tank forever changed the nature of war and expectation of how wars might be fought in the future. Like all of the major combatants, the United States Army attempted to capture the lessons of World War I as it considered how it would organize, train, and equip its forces for the future. Recognizing that the airplane had taken on the many of the strategic reconnaissance tasks once performed by horse cavalry, the United States Army concluded that some form of mechanized means might extend the range of horse cavalry units, still required for reconnaissance during poor weather and periods of limited visibility. Not as clear at the end of World War I, was that future tank forces might need their own reconnaissance forces. Although no serious efforts were taken to integrate mechanized reconnaissance vehicles into existing army formations until 1928, and then only on an experimental basis, few doubted that the mechanized reconnaissance units that might be developed would exist mainly to serve Cavalry Branch, still doctrinally and culturally dominated by a reliance on horses. Mechanized reconnaissance experimentation continued during the 1930s, at first largely in service of horses, but by the early 1940s in full competition with horses. Efforts to contain the role of mechanization to preserve a role for the horse cavalry were reflected in the doctrine and organization of mechanized reconnaissance units. Only after World War II began, did mechanized units fully usurp all of Cavalry Branch’s traditional missions to include reconnaissance. The test of war revealed a number of flaws in mechanized reconnaissance doctrine and organization. The unwillingness of former horse cavalry soldiers to abandon their former doctrine resulted in mechanized reconnaissance units far more combat capable than anticipated during the interwar years, but still incapable of fulfilling their anticipated reconnaissance duties. At the end of World War II, mechanized reconnaissance doctrine and organization sought to fully restore Cavalry Branch’s identity as a combat arm even without horses. Only those who fought the hardest to maintain a role for the horse before World War II considered trying to restore horses to Cavalry Branch after World War II. The horse advocates were unsuccessful and the Army retains fully mechanized reconnaissance organizations shaped not only by interwar experimentation, but by the World War II experience.